Experimental investigation of the operating pressure of an ammonia/water absorption heat transformer
Absorption heat transformers (AHT) are thermal energy converters that elevate heat from a medium to a higher, usable temperature level. To achieve this, some of the heat is dissipated at a lower temperature. The driving energy is primarily supplied as heat at the medium temperature level, while the requirement for electrical energy remains minimal. Therefore, AHT can serve as an alternative to high-temperature compression heat pumps. In this context, they represent a promising technology for decarbonizing the heat generation of industrial processes. While various working pairs have been studied, research on ammonia/water (NH3/H2O) AHT remains limited. Only a few experimental setups have been documented in the literature. This study presents experimental results obtained from a laboratory-scale NH3/H2O AHT, delivering heat at temperatures up to 120 °C. The AHT features a continuously adjustable expansion valve, allowing for precise control of the high-pressure level during operation. This study examined the relationship between internal process variables and system pressure levels. One significant drawback often associated with the NH3/H2O working pair is its high operating pressure. However, this study demonstrates that the actual high-pressure level is considerably lower than that predicted by several simulation studies. It is shown that widespread modeling approaches overestimate the pressure requirements of AHT in practical applications and, therefore, limit perceived feasibility. In addition, the influence of the high-pressure level on both useful heat output and COP was analyzed. It is shown that both performance parameters peak at similar high-pressure levels for each operating point. A useful heat output of up to 7.6 kW with a COP of up to 0.37 was achieved at a temperature lift of 30 K.